This invention relates generally to improvements in pulverizers, including the separation of coarse particles from fine particles, an improved free air purging mechanical seal assembly provided between the classifier and housing, free air cooling of the drive shaft, and a feed distributer which facilitates the drying of wet particulate material.
Pulverizing of dry materials is practiced today using hammer mills, impact attrition mills, ball mills, and others outfitted with internal classifiers that separate the coarse and the fine particle fractions. Problems with these mills include inadequate internal circulation of material flow thereby causing excessive grinding, reduced classifier efficiency, lower throughput, and wearing of certain parts of the mill.
A typical impact attrition pulverizer, shown in FIG. 1, comprises vertically oriented cylindrical housing 2 provided with air inlet 3, feed inlet 4, and product outlet 6xe2x80x94coordinated with a suction device (not shown), coarse particle exit tube 8, and classifier 10. Conventional classifiers typically comprise a vaned wheel that generates a centrifugal air flow. A problem with the type of classifier shown in FIG. 1 is inefficient particle separation. More sophisticated classifiers capable of higher particle separation efficiency have been developed, but with these, particle buildup between the classifier and the housing necessitates frequent disassembly and cleaning and can cause mechanical failure.
Housing 2 has inner liner 28 that is preferably provided with a surface that facilitates pulverization, such as a plurality of ridges that extend parallel to the center line of housing 2. A drive shaft 24 is vertically oriented along the center line of housing 2. Drive shaft 24 coaxially supports rotor 30 and classifier 10. Rotor 30 preferably comprises a plurality of rotor segments 32. A plurality of spaced beater plates 34 reside around the circumference of rotor segments 32, such that the foremost edges of beater plates 34 extend radially toward inner liner 28 and align vertically with drive shaft 24. The rotor segments 32 may be separated from each other by partition disks 36. A particle pulverizing domain 38 is defined between beater plates 34, inner liner 28, as well as in the pocket formed by the beater plates and the partition disks.
During operation, a rotating device (not shown) rotates drive shaft 24 at high speed and the suction device at product outlet 6 pulls external air through the apparatus from air inlet 3 and feed inlet 4. The material to be processed is introduced at feed inlet 4 and is wafted through particle pulverizing domain 38. Within particle pulverizing domain 38, the periphery of rotor 30 (i.e., the edges of beater plates 34) and inner liner 28, cooperate to grind and pulverize the substrate material. The material is ground by impact with beater plates 34 and inner liner 28, as well as attrition between particles.
Classifier 10 allows the finer particles to pass through toward product outlet 6 and the coarse particles are rejected and directed toward the inner liner 28 by the centrifugal force generated by the classifier and thrown out from the coarse particle exit tube 8.
It is undesirable to permit over-sized particles to be discharged with the desired fine particle material at the product outlet. Such particles are regarded as contaminants and lower the quality of the product. It is common practice to remove coarse particles from the classifying zone and recycle them as tailings for further reduction, for example, via the coarse particle exit tube 8 shown in FIG. 1. But this arrangement is insufficient, especially for ultra-fine grinding. In the conventional device described above, removal of the coarse particles relies solely on the momentum of the coarse particle induced by the centrifugal force from the classifier. Thus, the coarse particles, rejected by the classifier, tend to accumulate around the classification zone and eventually leads to clogging and malfunction.
It is especially important to reliably discharge the coarse particle fraction if it includes a contaminant that is harder than the material being ground. It is well known that a small percentage of hard abrasive contaminants can greatly reduce the capacity of the pulverizing apparatus. Such contaminants also make it difficult achieve stringent top-size requirement. For example, limestone, depending on its source, may have a small percentage of alumina or magnesia scale. Since these particles are very hard, they cannot be fully pulverized and they will either continuously re-circulate through the pulverizer or be discharged with the fine particle fraction. Thus, improvements in extraction and grinding of the coarse particles are needed.
It is well known in the art that conventional pulverizers can be used to dry wet particulate matter slurries. In the adaptation of a pulverizer for drying applications, a wet material suspension is dispersed in the pulverization domain and mixed with hot turbulent air. The hot air is introduced into the pulverizer from air inlet 3.
Like other drying processes, under a given capacity, the higher the air temperature, the less air flow required. A problem with adapting conventional pulverizers for drying is overheating of the area where the drive shaft meets the lower bearing member. The overheating is caused by rotational friction and the hot inlet air. Thus, with conventional pulverizers in drying applications, there is a limitation on inlet air temperature and extra oil-cooling arrangements are required to prevent overheating of the area where the drive shaft meets the lower bearing member.
In a drying application, the wet substrate suspension should be introduced directly onto the rotor. This is necessary because a high degree of initial dispersionxe2x80x94provided by the action of the high speed rotor 30xe2x80x94is required for drying. However, the drying efficiency can further be enhanced by a proper design of the feed intake nozzle.
The present invention is directed to an improved device that alleviates these problems and provides features herebefore unknown in the art.
The invention generally relates to a particle pulverizing apparatus having a cylindrical housing oriented along a vertical axis. This housing is typically provided with an inner liner, an inlet, an outlet, and a rotatable drive shaft oriented along the vertical axis. The drive shaft supports a rotor having an outer diameter such that a particle pulverizing domain exists between the inner liner and the rotor, as well as in the pocket formed by the beater plates and the partition disks.
In one embodiment, a coarse particle extraction assembly is provided. This assembly includes an extraction port in the housing, a pipe positioned adjacent the extraction port and including an air nozzle therein. During operation, the air nozzle discharges an air jet into the pipe to generate a vacuum at the extraction port to extract the coarse particles from the housing through the extraction port. This coarse particle extraction system efficiently removes the coarse particle fraction of the particulate material for collection or further pulverization.
Advantageously, the pipe is configured and dimensioned to provide a non-linear path for the coarse particles, and the air nozzle is positioned such that the air jet can convey and accelerate the coarse particles. Preferably, an impact plate is positioned downstream of the air nozzle for directing the extracted coarse particles back into the housing. If desired, a venturi can be positioned between the air nozzle and the impact plate to guide return of the extracted coarse particles. As used herein, a venturi comprises a tube with a convergent section, a venturi throat, and a divergent section.
In another embodiment, the coarse particle extraction system may include an acceleration chute, positioned between the air nozzle and the impact plate for further accelerating and directing the coarse particles directly onto the impact plate for further grinding. The acceleration chute is a straight tube section extending immediately from a venturi throat.
In another embodiment, the pulverizing apparatus further includes a rotatable classifier positioned on the drive shaft above the rotor and a stationary free air purging mechanical seal assembly supported by the housing and positioned adjacent to the classifier. The classifier advantageously comprises a lip having a plurality of spaced fins thereon and a baffle extending perpendicularly thereto. The free air purging mechanical seal assembly advantageously comprises a stationary casing supported by the housing and having upper, lower, and inner walls defining a annular cavity therebetween. An aperture on the upper wall of the casing opens into a tube, which tube is connected with an air orifice in the housing to allow external air to enter the annular cavity. The lower wall is provided with air path, wherein rotation of the classifier induces air flow by the action of the fins, allowing the air in the annular cavity to pass through the air path over the upper surface of the classifier and into the housing to prevent particles from passing through the seal gap between the baffle and the inner wall.
Preferably, the lower wall includes a groove therein for receiving at least a portion of the classifier lip fins extending vertically.
The air path in the lower wall preferably comprises a plurality of equally spaced holes for controlling the distribution of air onto the upper surface of the classifier.
Preferably, the mechanical seal assembly further comprises an annular opening extending around the inner wall adjacent to the baffle and the lower portion of the inner wall of the casing preferably comprises notch to receive at least a portion of the baffle.
The classifier/free air purging mechanical seal arrangement of the invention prevents mechanical failure and frequent cleaning of the classifier seal required by conventional pulverizers.
In yet another embodiment, the pulverizing apparatus further includes a free air cooling arrangement comprising a stationary sleeve surrounding the drive shaft under the rotor, the stationary sleeve forming a annular duct around the drive shaft for providing a first path for external air to flow through the annular duct into the housing for cooling the drive shaft. Preferably, a rotatable sleeve, rotatably supported by the drive shaft surrounds the stationary sleeve creating an annular void therebetween that provides a second path for the external air to flow into the housing when the apparatus is in operation. This arrangement eliminates the need for extra oil cooling systems.
In still another embodiment, the particle pulverizing apparatus may include a wet material feed intake nozzle, preferably comprising a slot shaped hole located on the housing near the rotor for dispersing a suspension of wet particulate material onto the rotor. Preferably, the slot shaped hole is parallel to the axial direction of the housing.